1. Field of the Invention
The present invention relates to a developing device used for an image forming apparatus, such as a copy machine or a printer, and a method of forming an image and, more specifically, relates to a developing device included in an image forming apparatus using a color toner and a transparent toner according to a two-component development method.
2. Description of the Related Art
For a known image forming apparatus employing electrophotography and, more specifically, for an image forming apparatus configured to form images in chromatic colors, a two-component development method using a developer including a mixture of nonmagnetic toner particles and magnetic carrier particles is widely used.
A two-component development method, compared to other development methods used today, is advantageous in that the image quality is stable and the apparatus is highly durable for long-term use. However, the two-component development method is disadvantageous in that the developer is degraded through use and developability changes due to a reduction in the electrostatic charge (hereinafter referred to as “triboelectricity”) caused by degradation of carrier particles and defects in the printout images, such as a change in color as the number of image printouts increase and toner scattering. For these reasons, when the image forming apparatus is to be used long term, down time (time period in which the apparatus cannot be used for printing out images due to adjustment of the apparatus) and man-hours for replacing the developer are required.
Japanese Patent Publication No. 2-21591 discloses a method of reducing the man-hours required for replacing the developer by continuously collecting the degraded developer in small amounts and continuously supplying new developer with the same amounts while maintaining the performance of the developer at a predetermined level. More specifically, by gradually replacing the degraded developer (carrier particles) with new developer, apparent degradation of the carrier particles is prevented, the total volume of the developer is stabilized, and automatic replacement of the developer is substituted for manual replacement.
Recently, in the print-on-demand (POD) market, there has been an increasing need in printing out stable images using electrophotography employing the two-component development method while minimizing down time. To satisfy this need, technology such as that disclosed in Japanese Patent Publication No. 2-21591 is useful. By employing such technology, the degradation of the developer can be stabilized at a predetermined level to prevent a change in image quality due to the degradation of the developer.
Degradation of carrier particles can be defined by a reduction in ability of the carrier particles to apply triboelectric charges to the toner particles. More specifically, the carrier particles gradually degrade, or gradually lose their ability to apply triboelectric charges to the toner particles, when the coating agent covering the surfaces of the carrier particles is scraped off and/or toner particles and additive particles cling to the surface of the carrier particles.
By employing the technology described in Japanese Patent Publication No. 2-21591, degradation of the carrier particles contained in a developing unit can be suppressed. This is possible because, the degradation level of the carrier particles can be changed by changing the frequency of replenishment and drainage of the carrier particles based on the number of printouts made.
More simply, if the carrier particles are replaced frequently, the developer will stay in a relatively fresh state. Now, the difference in the levels of degradation based on image ratio will be described.
The “age” of the carrier particles, i.e., the amount of time each carrier particle is used in a developer container, is represented by printouts, i.e., the number of images printed out on A4-size recording sheets. In a durability test, x represents the number of printouts, P(x) represents the average age of the carrier particles in a developer container, and W(g) represents the total amount of carrier particles in the developer container. Moreover, d(g) represents the amount of new carrier particles that are replenished when toner is consumed to make one printout and also represents the amount of developer that is drained from the developer container as the new carrier particles d(g) are replenished.
For calculation, if it is assumed that image formation and carrier particle replenishment is carried out time-sequentially, the following formula holds:Q(x)=P(x)×[(W−d)/W]+P(0)×[d/W]  (1)Wherein, P(x) represents the average age of the carrier particles immediately after forming x printouts and immediately before replenishing the carrier particles, and Q(x) represents the average age of the carrier particles immediately after replenishing the carrier particles. Here, since P(0) is the average initial age of the carrier particles, P(0)=0, and, therefore:Q(x)=P(x)×[(W−d)/W]  (2)
P(x+1) represents the average age after one printout is made at Q(x). If it is assumed that the carrier particles are used equally in forming the printout, then, the following formula holds:P(x+1)=Q(x)+1  (3)Based on formulas (2) and (3):P(x+1)=P(x)×[(W−d)/W]+1  (4)P(x)=[1−(1−d/W)x]×W/d  (5)
In other words, the average age of the carrier particles when the developer is automatically replaced converges to W/d (total amount of carrier particles in developer container/amount of replaced carrier per printout).
More specifically, for example, if the weight of the developer in the developer container is 375 g and the toner concentration in the developer in the developer container (i.e., proportion of the weight of the toner particles to the total weight of the developer (hereinafter referred to as the “TD ratio”)) is 8%, the weight of carrier particles is 345 g. The proportion of the weight of the carrier particles to the total weight of the developer supplied for replenishment (hereinafter referred to as “replenishment developer”) is 15% (this proportion is referred to as the “CD ratio”). For example, if 0.7 mg/cm2 is the amount of toner particles that need to be applied to a recording sheet to obtain the maximum density, when the image ratio is 5%, 21.3 mg of toner is consumed per A4-size recording sheet. At this time, the amount of carrier particles replaced per recording sheet is 3.8 mg. The calculation results based on this information are shown in FIG. 3 as a graph illustrating the change in average age.
The dotted line in the graph represents the result when the CD ratio of the replenishment developer is 0%, i.e., when the amount of carrier particles is 0. In this case, the number of printouts made and the average age of the carrier particles are the same. Moreover, FIG. 3 shows the results when the image ratio is 10% and 50%.
As shown in FIG. 3, by using a replenishment developer having a CD ratio of 15%, when 300K (300,000) printouts are made with an image ratio of 5%, the average age of the carrier particles is stabilized at 90K printouts. Whereas, by using a replenishment developer having a CD ratio of 0%, when 300K printouts are made with an image ratio of 5%, the average age of the carrier particles is 300K printouts wherein replacement of the developer is required.
In this way, by draining the carrier particles from the developer container and replenishing new carrier particles together with new toner particles, the degradation level of the carrier particles in the developer container can be suppressed.
In response to the recent increase in need for high-quality images, technology for improving image quality has been proposed. Such technology includes an inkjet image forming apparatus configured to printout photographic-quality images using five or more ink colors. Furthermore, for an image forming apparatus employing electrophotography, technology for achieving high image quality by improving the half tone gradation by using multi-color development (development of five or more colors) and improving the glossiness of the surface of the recording sheet by fixing a transparent toner on the uppermost layer of the sheet has been proposed.
For example, Japanese Patent Laid-Open No. 4-278967 (corresponding to U.S. Pat. No. 5,260,753) discloses technology for improving the glossiness of the image surface by developing the entire image formation area with a transparent toner so as to provide a color image having a color tone similar to a silver photograph.
Japanese Patent Laid-Open Nos. 5-6033, 5-127437, and 2000-147863 disclose technologies for, not only improving the glossiness of the image surface by developing the entire image formation area with a transparent toner, but also providing an image even more similar to a silver photograph by adjusting the amount of transparent toner applied to the surface of the recording sheet so as to form a uniform surface with less unevenness caused by accumulation of the toner.
However, when using both a color toner and a transparent toner in a two-component development method, the following problems have been discovered.
Any development method using the above-described transparent toner applies transparent toner to the entire image to develop a substantially solid image. Therefore, each time an image is printed out, the transparent toner consumed in forming the solid image must be replenished.
Therefore, for example, when multiple printouts are made, a large quantity of toner is repeatedly replenished, causing development to be carried out with toner that has been insufficiently charged. If the toner is insufficiently charged, the triboelectric charge is lowered, causing problems, such as toner scattering inside the apparatus and fogging. Such problems may be solved by extending the stirring path of the developer inside the developing unit, i.e., the length from the developer inlet to the outlet where the developer is supplied for development or by increasing the volume of the developer in the developing unit so that the toner supplied to the developing unit is sufficiently charged before the toner reaches the developer bearing member configured to deliver the toner to the opposing area (development area) of the image bearing member. However, such method causes an increase in costs since the size of the developing unit is increased and the structure of the developing unit becomes complicated. Furthermore, there is another problem in that when the image ratio is high and the toner in the developing unit is replaced frequently, accumulation of additive particles (attachment of the additive particles to the surface of the carrier particles and/or the additive particles being released) accelerates the degradation of the carrier particles and may cause significant reduction in the triboelectric charge of the toner and developability. A change in developability may cause defective images with color change and/or toner scattering.
In other words, such as the transparent toner developing unit, when images having a high image ratio are printed out repeatedly, the large amount of toner consumed as compared to when images having a low image ratio are printed out, the number of toner replenishment increases. Therefore, the amount of carrier particles replenished to the developing unit also increases. When the image ratio is high, accumulation of the additive particles (attachment of the additive particles to the surface of the carrier particles and/or the additive particles being released) becomes significant.
Furthermore, FIG. 4 shows a graph of calculated results and experimental results of the average age of the carrier particles when a replenishment developer having a CD ratio of 10% is used and the image ratios of the printouts are 10% and 30%. When the image ratio is either 10% or 30%, a correlation between the calculated result and the experimental result is recognized.
In contrast, FIG. 5 shows the calculated result and the experimental result of the average age of the carrier particles when a substantially solid image is developed using a replenishment developer having a CD ratio of 10% at an image ratio of 70% in accordance with the example of transparent toner usage. In this case, the calculated result and the experimental result do not match at all.
Here, the actual average age of the carrier particles (experimental result) is determined by measuring the ability of charging the toner when the carrier particles and the toner particles tested for durability are mixed under predetermined conditions and by comparing this with the charging ability of the initial carrier particles and the replaced carrier particles. Furthermore, the average age of the carrier particles can be determined by comparing the surfaces of the aged carrier particles with initial carrier particles for the amount of additive particles attached to the surfaces and/or scratches and unevenness of the surfaces.
When the inside of the apparatus used for the experiment was checked after conducting the durability test with an image ratio of 70%, intensive scattering of the transparent toner was observed inside the apparatus. During the durability test, after about 150K printouts, a decrease in the triboelectric charge of the toner due to degradation of the developer was observed at the transparent toner developing unit. This decrease caused excess amounts of transparent toner to be applied to the recording sheet and defective fixing and jamming of the recording sheets due to defective conveying to occur.
When the property of the developer in the transparent toner developing unit was checked after the durability test conducted at an image ratio of 70% was completed, the triboelectric charge of the initial toner was 37 μC/g, whereas the triboelectric charge of the toner after completion of the durability test was 18 μC/g, which is about half of that of the initial toner. Moreover, the amount of additive particles in the developing unit had significantly increased, and the additive particles had attached to the surface of the carrier particles and/or had been released.
In other words, when printouts with a high image ratio are repeatedly outputted, the amount of carrier particles replenished to the developing unit increases as the number of toner replenishment increases. As a result, the average age of the carrier particles in the developing unit is lowered. However, when the image ratio is increased to about 70%, the effect of the degradation of the carrier particles due to accumulation of the additive particles (attachment of the additive particles to the surface of the carrier particles and/or the additive particles being released) surpasses the effect of the renewal of the carrier particles by replacement.
In other words, when the image ratio is low, the following relationship holds:renewal of carrier particles by replacement>degradation due to accumulation of additive particles.As shown in FIG. 4, there is a correlation between the calculated results and the experimental results of the average age of the carrier particles.
However, when the image ratio is high, such as in the above-described case where transparent toner is used, the following relationship holds:degradation due to accumulation of additive particles>renewal of carrier particles by replacementWherein, the actual developer degrades significantly faster than the theoretical estimate. As a result, as shown in FIG. 5, the calculated result and the experimental result of the average age of the carrier particles do not match at all.
Therefore, when the image ratio is high, the replenished toner is insufficiently charged because the developer is degraded even though the carrier particles are being replaced. As a result, toner scattering, fogging, and/or defective fixing due to excess application of the toner onto the recording sheet may occur.
Moreover, the increase in the toner causing fogging may increase the load applied on the cleaning member, causing defective cleaning. Moreover, in case an optical sensor is used to read the amount of light reflected from the photosensitive body or the intermediate transfer body, fogging may cause a change in the detected amount of reflected light, causing erroneous detection and/or erroneous operation of the sensor.
To avoid such above-described problems, down time and man-hours for replacing the developer are required when the image forming apparatus is used long term.